The present invention relates to an internal combustion engine for a vehicle, and more particularly, to an internal combustion engine improved in the charging efficiency of an air-fuel mixture to each cylinder of the engine.
In a conventional four-cycle internal combustion engine for an automobile comprises one intake valve and one exhaust valve for each cylinder or combustion chamber. In order to enhance the output performance of the engine of this type, the charging efficiency of an air-fuel mixture to the combustion chambers and the discharging efficiency of exhaust gases therefrom must be improved. To attain this, an intake port and an exhaust port of each combustion chamber, adapted to be opened and closed by means of the intake valve and the exhaust valve, in each engine cylinder, should preferably be maximized in size. Usually, however, the intake and exhaust ports, as well as the bore of the cylinder, are circular in shape, so that their maximum permissible size is restricted by the diameter of the cylinder bore. Accordingly, there are some conventional engines in which the number of intake and exhaust ports for each cylinder is increased in order that the total opening areas of the ports are large enough even though the opening area of each port is reduced.
One such conventional engine comprises, for example, two intake ports and two exhaust ports for each cylinder and twin camshafts. The engine of this type, having two intake valves and two exhaust valves for opening and closing the intake and exhaust ports, respectively, are called a four-valve engine.
As compared with the two-valve engine, having one intake valve and one exhaust valve for each cylinder, the four-valve engine can enjoy improved output performance, higher rotating speed due to reduction of the weight of valve drive mechanisms, and less mechanical loss. Also, the low- and medium-speed torque performance can be improved by controlling the valve timing. Thus, the four-valve engine has started to be used as a practical engine, as well as a high-output engine for a sports car or the like.
There has recently been a demand for an engine whose output can be made higher than the four-valve engine, and in which the amount of fuel supply to the combustion chambers can be controlled in accordance with the operating conditions of the engine.
In developing the engine of this type, the engine output may be further improved by increasing the number of intake valves used in the engine to five. In this case, the developed engine is a five-valve engine. In order to control the fuel supply to the combustion chambers in accordance with the operating conditions of the engine, moreover, the operation of fuel injection valves, which are used to inject fuel directly into an intake manifold connecting with the individual combustion chambers, may be controlled by means of an electronic control device. The electronic control device, which includes a programmable electronic circuit such as a microcomputer, serves to determine the operating conditions of the engine in accordance with signals from various sensors, and control the operation of the fuel injection valve so that the air-fuel mixture can enjoy an optimum air-fuel ratio depending on the operating conditions.
In a specific example of the aforementioned internal combustion engine, the lower-course region of the inside of an intake passage leading to each cylinder is divided into three branch intake passages, which are connected individually to intake ports adapted to cooperate with their corresponding intake valves, and one fuel injection valve is disposed in a region on the upper-course side of the branch intake passages of each intake passage.
FIGS. 1 and 2 show a conventional one-flow injection valve 10 which has one jet 10a, and is used in the internal combustion engine of the aforesaid type. In the injection valve 10, an atomized fuel flow 11 injected from the jet 10a is supplied to three intake ports, including a central intake port 13 and two outside intake ports 12 and 14.
FIGS. 3 and 4 show a conventional two-flow injection valve 15 which has two jets 15a and 15b. In the injection valve 15, two atomized fuel flows 16 and 17 are injected from jets 15a and 15b, respectively. The one fuel flow 16 is supplied to the one outside intake port and one half of the central intake port 13, while the other fuel flow 17 is supplied to the other outside intake port 14 and the other half of the central intake port 13.
In the engine having the fuel injection valves of this type, however, fuel injected from each fuel injection valve has the form of one or two atomized fuel flows which radially spread toward the branch intake passages. It is difficult, therefore, to distribute the atomized fuel flow or flows uniformly to the three branch intake passages. Thus, air-fuel mixtures fed individually through the intake ports into each combustion chamber are different in fuel concentration. In consequence, the fuel concentration distribution in the combustion chambers is uneven, so that the fuel cannot undergo perfect combustion.
FIGS. 5 to 7 show a free-flow injection valve 18, disclosed in Japanese Utility Model Disclosure No. 61-186726, which has three jets 18a, 18b and 18c. The jets 18a, 18b and 18c of the injection valve 18, which serve to inject fuel toward intake ports 12, 13 and 14, respectively, are arranged in a straight line, and open so that the central jet 18b enjoys the largest injection quantity. Having different opening areas, the jets 18a to 18c are intended positively to cause unevenness in the fuel concentration distribution. In the engine disclosed in Japanese Utility Model Disclosure No. 61-186726, an intake control valve (not shown) is disposed in the intake port 12. The control valve is opened and closed during high- and low-load operations of the engine, respectively, so that the low-load combustion performance is improved. When the intake control valve is closed, however, a greater amount of fuel adheres to the wall surface near the intake port in which the control valve is located, so that the air-fuel ratio of the air-fuel mixture introduced through the central intake port 13 is excessively fuel-rich. Also when the intake control valve is open, only the air-fuel mixture from the central intake port 13 is excessively fuel-rich.
The fuel supplied to the combustion chambers is also utilized for cooling the intake valves. If the atomized fuel flows passing through the individual branch intake passages are different in fuel concentration, however, the intake valves cannot be cooled uniformly.
As mentioned before, moreover, the atomized fuel flows from each fuel injection valve radially spread toward the branch intake passages, so that the amount of fuel adhering to the respective inner walls of the branch passages naturally increases. Accordingly, the necessary fuel amount cannot be secured for acceleration. If the fuel supply is interrupted at the time of deceleration, on the other hand, the fuel adhering to the wall surface flows into the combustion chambers, thus exerting a bad influence on the responsiveness of the engine.